1. Field of the Invention
The present invention relates to a spark plug manufacturing apparatus provided with a holding plate (pallet) for holding insulators for spark plugs during a heating process for manufacturing the spark plugs, and to a method of manufacturing spark plugs by use of the holding plate.
2. Description of Related Art
Generally, a spark plug for an internal combustion engine includes a tubular mounting fitting having a threaded portion for installation to the engine, an insulator fixed to the inside of the mounting fitting such that the front end thereof projects from the front end of the mounting fitting, a center electrode fitted into an axial hole of the insulator such that the front end thereof projects from the front end of the insulator, and a ground electrode fixed to the mounting fitting so as to face the front end of the center electrode across from a spark discharge gap.
First and second glass seal layers for providing the axial hole of the insulator with air tightness, and a resistor glass are provided in the axial hole. The rear end of the center electrode is electrically connected to the resistor glass through the first glass seal layer. The resistor glass is electrically connected to one end of a metal stem through the second glass seal layer at the side of the rear end of the insulator within the axial hole. A terminal part which exposes at the surface of the rear end of the insulator is connected to the other end of the stem. This terminal part is fitted with a boots part of an ignition coil.
Next, explanation is made as to how the first and second glass seal layers, and the resistor glass are formed in the axial hole of the insulator (for detail, refer to Japanese Patent Application Laid-open No. 2004-319335, for example).
At the beginning, the center electrode is fitted into the hollow portion (axial hole) of the hollow tubular insulator. After that, a powder material of a conductive glass is charged into the hollow portion and pressurized to make the first glass seal layer in a first glass material charging process.
Subsequently, a resistor material of the resistor glass is charged into the hollow portion and pressurized on the powder material charged in the first glass material charging process. Next, a powder material of a conductive glass is charged into the hollow portion and pressurized by the stem to make the second glass seal layer in a second glass material charging process.
After that, a plurality of the insulators each of which has undergone the above described processes are loaded on a pallet which is made of a heat-resisting steel and exhibits resistance to thermal shock applied in a heating process where quick heat-up and rapid cool-down are repeated. This pallet has a plurality of mounting holes to which the insulators are fitted so that they can be heated at once in order to increase the productivity of the spark plugs.
In a subsequent heating process, the pallet is carried into an electric furnace where the insulators loaded on the pallet are heated for a certain time at a certain temperature, for example, at 900 degrees C. FIG. 10 shows the insulators 20 loaded on the pallet 500 carried into the electric furnace. In this heating process, the pallet 500 slides over a hearth 600 to move in the electric furnace while being heated by an upper electric heater 710 facing the top surface of the pallet 500 and a lower electric heater 720 facing the rear surface of the pallet 500. Inconsequence, the insulators 20 are heated and the first and second glass seal layers of each insulator 20 are put in a molten state.
After completion of the heating process, the pallet 500 is carried out from the electric furnace, and the terminal parts 71 are pressed down into the interiors of the insulators 20. After that, the insulators 20 are cooled down rapidly, as a result of which the first and second glass seal layers and the resistor glass of each insulator 20 are solidified. In this way, the first and second glass seal layers and the resistor glass are formed in the insulator 20.
However, the inventor has found that the pallet 500 used in the heating process has technical challenges to be resolved, which are set forth below.
First, the pallet 500, which is capable of loading a plurality of works (insulators 20) thereon to increase the productivity, is heavy in weight, because it is made of the heat-resisting steel. Accordingly, prior to heating the works to 900 degrees C., the pallet 500 has to be heated. However, because of the heavy weight of the heat-resisting steel (more than 4 kg/50 pieces of works (mounting holes), for example), heating the pallet takes a long time and a large amount of energy.
Secondly, since the pallet 500 is subjected to cycles of quick heating up and rapid cooling down during the heating process, the pallet 500 is oxidized, and is deformed due to thermal expansion. Accordingly, the life span of the pallet 500 is as short as from one year and a half to two years.
Thirdly, at the time of pressing down the terminal part 71, the terminal part 71 may be off the center of the axis of the insulator 20 due to deformation of the pallet 500. If the offset value is too large, there is a possibility that the insulator 20 is broken.
Fourthly, it takes a long time for the temperature of the pallet 500 to become uniform since the pallet 500 is made of the heat-resisting steel having a low thermal conductivity. In addition, when the pallet 500 is heated at a plurality of different positions thereof independently, there arises position-related temperature difference. The inventor has found through experiment that the interior temperature difference between the insulator 20 located at the edge portion of the pallet 500 and the insulator 20 located at the center portion of the pallet 500 is more than 80 degrees C./50 pieces. Such a large temperature difference causes the resistor glasses of the insulators 20 to have different resistances even though they have been loaded on the same pallet and subjected to the same heating process.